Welcome to Journal Citations Reports
Ophthalmology falls under the Medicine category. "SNIP measures a source’s contextual citation impact by weighting citations based on the total number of citations in a subject field."
Google Scholar also collects journal metrics (h5-index and h5-median).
"h5-index is the h-index for articles published in the last 5 complete years. It is the largest number h such that h articles published in 2013-2017 have at least h citations each."
"h5-median for a publication is the median number of citations for the articles that make up its h5-index."